This invention relates to magnet based position sensor. More specifically, this invention relates to a position sensor which uses a magnet which is magnetized such that the angle of the magnetic field varies linearly when rotated or translated.
It is desirable in many applications to determine the precise position of various objects which rotate. This is accomplished using magnetically based sensors. Such sensors function by measuring either the change in magnetic field intensity such as by a Hall effect sensor or the change in the angle of the fields such as an anisotropic magneto-resistence (AMR) sensor. A magnet is placed on the axis of a rotating element and rotation angle is sensed by the sensor according to the field angle. Such sensors provide cost effective and accurate measurement of the rotational angle of the object through the measurement of the field intensity or field angle.
Unfortunately, there are numerous applications in which a magnet cannot be placed on the axis of rotation. For example, a sensor for steering wheel position must be mounted on the outer surface of the steering wheel shaft. In such-applications, the magnet is installed on the periphery of the device. However, since the magnet is not on the axis of rotation, the path transcribed by the magnet does not produce an easily quantifiable relationship between input angle and the output field angle. In such applications, some form of field shaping must be employed by the magnetic field sensor in order to modify the field gradient from the magnet such that a relationship may be generated between the rotation and the output voltage of the sensed magnetic field. Such devices also must be used for a magnetic sensor which senses the location of objects which linearly traverse a defined path, since the magnitude of the field changes disproportionally with the distance.
Such present methods suffer from several problems. Most significantly, the magnetic field strength nor the angle of the magnetic field are exactly linear which introduces error in the position measurement. This inaccuracy increases as the magnetic field reach further distances away from the transducers for measurement of the fields.
Additional solutions have included using a linear magneto-resistive transducer in conjunction with a moving magnet. The magnetic field sensed by the transducers is an indication of the position of the magnet. However, the non-linear nature of the magnetic field results in distortions near the ends of the traversal of the magnet. Such distortions may be corrected, but such corrections require extra circuitry or processing which increase the complexity and cost of the device. Additionally, it requires complex shaping of a magnet in order to insure proper magnetic field output over the range of movement of a measured object. Such shaping is difficult to achieve and adds to the cost of the sensor.
Thus, there exists a need for a magnetic sensor which allows placement of a magnet in the target object without reliance on placement on the rotational axis. There is also a further need for a position sensor which uses magnet which can be compensated for a non-axial location on the target object.
The present invention is embodied in a position sensor system for determining the position of a moveable object. The system has a magnet coupled to the object. A first magnetic field transducer detects the generated magnetic field and outputs a first sinusoidal signal representative of the magnetic field direction. A second magnetic field transducer detects the generated magnetic field and outputs a second sinusoidal signal representative of the magnetic field direction. A signal processor unit is coupled to the first and second magnetic field transducers, the signal processor unit outputs a signal which is a function of the sinusoidal signals representative of the position of the object relative to the first and second transducers. The magnet is magnetized such that the change of the angle of the generated magnetic field detected by the first and second magnetic field transducers is linear to the position of the magnet
The invention is also embodied in a method of determining the position of an object. A magnet is fixed on the object. The magnetic field direction produced by the magnets is detected. The magnetic field direction is converted into sinusoidal signals. The position of the object is determined based on the sinusoidal signals. The magnet is magnetized such that the change of the angle of the generated magnetic field detected is linear to the position of the magnet.
It is to be understood that both the foregoing general description and the following detailed description are not limiting but are intended to provide further explanation of the invention claimed. The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain the principles of the invention.